When you are on your uprank but Psyonix riggs it.

It's not rigged. When you're on a win streak, the game gives you harder opponents. If you win, you gain a lot of points and rank up much faster. If you lose, you lose very few points and are given easier opponents again. This is intentional and is a good thing for you.

Mate I never expected to play a fucking grand champion. I have never been higher than Chall 3 and when I just need that last little points this happens. The MM is a good thing I know I just lost a little bit of faith when I saw that yellow line at the start of a game.

You know that they wiped all ranks at the beginning of this season, right? Everyone started from scratch, and it takes time for people to settle back into their deserved rank. There's nothing "rigged" about it, and the situation will continue to improve as these former Champions play more games. Keep in mind that everyone else at your level is also hitting these occasional mismatches, so you're still in the right spot relative to your peers.